High-purity copper sputtering targets and copper alloy sputtering targets are currently used in a variety of applications including, for example, fabrication of integrated circuits. The quality of copper-comprising structures such as interconnects and thin films can depend upon sputtering performance of the target. Various factors of a sputtering target can influence the target's sputtering performance including: average grain size and grain size uniformity of the target material; crystallographic orientation/texture of the target material; structural and compositional homogeneity within the target; and the strength of the target material. Typically, a smaller average grain size is associated with an increased strength of material. Additionally, the amount of alloying can affect strength and hardness of target materials, with increased alloying typically resulting in increased target strength.
Due to the low strength of high-purity copper (greater than 99.99% copper by weight) conventional high-purity copper sputtering targets are typically formed as bonded targets. A bonded copper sputtering target can have a high-purity copper target bonded to a backing plate comprising a relatively high strength material such as, for example, aluminum. However, the high temperatures utilized during bonding of the copper target to the backing plate often results in abnormal grain growth resulting in non-uniformity of microstructure and an increase in overall average grain size. Conventional high-purity copper targets typically have an average grain size greater than 50 microns which can result in relatively low yield strength. The resulting grain size and structural non-uniformity of conventionally formed high-purity copper sputtering targets can detrimentally affect the quality of sputter-deposited high-purity copper films and interconnects.
In addition to the resulting large grain size and anomalous grain growth that can result during bonding processes, diffusion bonded copper targets are often plagued by problems such as burn through and short target life. Additionally, bonding processes can be complicated and time consuming.
One approach to increasing grain size uniformity and enhancing strength of copper materials for sputtering target purposes is to alloy the copper with one or more “alloying” elements. However, since the presence of alloying elements affects the resistivity of copper, it can be desirable to limit the total amount of alloying elements within a target material to no greater than 10 percent by weight. For particular applications such as copper thin films and interconnects, where a resistivity comparable to that of high-purity copper is desired, the amount of alloying should be limited to less than or equal to 3% by weight. Another draw back to alloying can be potential defects such as formation of second phase precipitates or segregation.
Although treatment of conventional materials for reduction or removal of precipitates or segregation defects may be possible in some instances, such treatment typically includes high temperatures which can result in extremely large grain sizes (greater than 150 microns). Alternatively, a partial reduction of second phase precipitates or segregation defects present in conventional materials can be obtained in some instances utilizing conventional rolling and/or forging techniques. However, the remaining defects can still affect the quality of sputtered films. Currently, conventional processing to form copper alloys having less than or equal to 3% by weight of alloying elements result in targets typically having an average grain size of over 30 microns, commonly over 50 microns, and having second phase precipitates therein.
It is desirable to develop methods to produce copper sputtering targets and copper alloy sputtering targets having improved sputtering performance.